A monograph of living New World monkeys in 3 volumes has been in preparation by the PI during the last 20 years. Volume 1, published 1977, deals with Primates generally, New World marmosets and tamarins of the family Callitrichidae, and the callimico, family Callimiconidae. Partially completed Volume 2 treats the non-prehensile-tailed monkeys of the family Cebidae, and Volume 3 the prehensile-tailed members of the same family. Volume 2 is designed as a complete reference work on the classification, interrelationships, geograhic distribution, origin, evolution, comparative and functional anatomy, genetics, serology, parasitology, reproduction, growth, development, behavior and conservation of the non-prehensile-tailed cebids Saimiri, Aotus, Callicebus, Pitchecia, Chiropotes and Cacajao with their combined 50 species and subspecies. Saimiri Aotus, Callicebus are the most commonly used New World monkeys in biomedical and behavioral research. Completed are the taxonomies of all the included species and subspecies with descriptions, comparisons, keys to identifications, synonymies, bibliographies, range maps, gazetteers of collecting and recorded localities, discussion on tegumentary pigmentation, pelage, sexual dimorphism including dichromatism, karyology, serology, and behavioral traits of taxonomic significance. Much of the information concerned with newly described or renamed taxons and revised intrageneric classifications have already been published, or are in press, as preliminary taxonomic reports. Most of the work on comparative morphology of phylogenetic significance or evolutionary importance to be completed will be derived from studies on prepared caracasses and complete skeletons. Consultants will be engaged opportunely for cytogenetic and serological research. Additional data on these and other topics will be compiled from the published literature, analyzed, synthesized and presented under appropriate chapter or section headings. Estimated length of the completed manuscript of Volume 2 is between 2000-3000 pages in addition to hundreds of illustrations, graphs, tables, charts and range maps. Present organization is by separate genera. Introductory chapter on relevant biological principles will be added on completion of the generic and contained species accounts.